Gelatin is used as a binder in many photographic light-sensitive materials. For example, each of a silver halide light-sensitive emulsion layer, an emulsion protective layer, a filter layer, an intermediate layer, an antihalation layer, a backing layer, a film base subbing layer, and a baryta layer contain gelatin as a major component.
Light-sensitive materials containing gelatin are processed with various aqueous solutions having various pH and temperature values. In such processing, gelatin is a major component determining the physical properties of a layer containing gelatin which is not treated with a hardening agent. Accordingly the layer is poor in water resistance and excessively swells in an aqueous solution, resulting in a serious reduction in its mechanical strength. Particularly when the material is processed in an aqueous solution maintained at temperatures as high as more than 30.degree. C. or in an aqueous solution having a high alkali content, in extreme cases, the gelatin layer is dissolved and flows out of the material. These properties are critical disadvantages for the photographic light-sensitive material.
A number of compounds have been known to be effective in increasing the light resistance, heat resistance, and mechanical strength of a gelatin layer by hardening gelatin therewith.
These compounds are used as hardening agents in the preparation of photographic light-sensitive materials. Typical examples are aldehyde-based compounds such as formaldehyde and glutaraldehyde; compounds containing a reactive halogen atom as described, for example, in U.S. Pat. No. 3,288,775; compounds containing a reactive ethylenically unsaturated bond as described, for example, in U.S. Pat. No. 3,642,486 and Japanese Patent Publication No. 13563/74; aziridine-based compounds as described, for example, in U.S. Pat. No. 3,017,280; epoxy compounds as described, for example, in U.S. Pat. No. 3,091,537; halogen carboxylaldehydes such as mucochloric acid; dioxanes such as dihydroxydioxane and dichlorodioxane; and inorganic hardening agents such as chromium alum and zirconium sulfate.
These known gelatin-hardening agents, however, when used in a photographic light-sensitive material, give rise to various problems. Some of the hardening agents are poor in the hardening effect. Some hardening agents suffer from a variation in the degree of hardening over a long period of time, which is called "post-hardening", due to their slow hardening of gelatin. Some hardening agents exert adverse influences on properties of the photographic light-sensitive material (particularly, an increase in fog and a reduction in light sensitivity), or lose their hardening capability under the influences of other coexisting photographic additives, or reduce the effect of other photographic additives (e.g., a coloring agent for color light-sensitive materials).
Hardening agents having a relatively high speed for hardening gelatin and less tendency for causing post-hardening include compounds having a dihydroquinoline skelton as described in Japanese Patent Application (OPI) No. 38540/75 (corresponding to U.S. Pat. No. 4,013,468) (the term "OPI" as used herein means a "published unexamined Japanese patent application"), compounds having a phosphorus-halogen bond as described in Japanese Patent Application (OPI) No. 113929/83, compounds having an N-sulfonyloxyimide group as described in Japanese Patent Application (OPI) No. 93470/77 (corresponding to U.S. Pat. No. 4,111,926), compounds containing at least two N-acyloxyimino groups in the molecule thereof as described in Japanese Patent Publication No. 22089/78 (corresponding to U.S. Pat. No. 4,052,373), N-carbamoylpyridinium salts as described in Japanese Patent Application (OPI) Nos. 51945/74 (corresponding to U.S. Pat. No. 3,880,665) and 59625/76 (corresponding to U.S. Pat. No. 4,063,952), 2-sulfonyloxypyridinium salts as described in Japanese Patent Application (OPI) No. 110762/81, and halothiazolium salts as described in Japanese Patent Application (OPI) No. 26844/72.
These hardening agents have advantages in that their gelatin-hardening speeds are high and they less cause post hardening. However, some of them are readily decomposed by the action of water; that is, their gelatin-hardening speeds are high while at the same time the side reaction that they are decomposed by water occurs at a high speed. Accordingly they have disadvantages in that in the usual method of preparation using an aqueous gelatin solution, the efficiency of effectively using the hardening agent is extremely low, and in order to obtain a gelatin coating having a desired degree of hardening, it is necessary to use a large amount of the hardening agent. On the other hand, some of them release acid in a large amount to decrease the pH of a photographic emulsion layer; consequently, they adversely affect to photographic properties.
It is generally believed that hardening agents as described in Japanese Patent Application (OPI) Nos. 51945/84, 59625/76 (corresponding to British Patent No. 1,383,630 and U.S. Pat. No. 4,063,952, respectively), and 110762/81 react with a carboxyl group and an amino group of gelatin under the nucleophilic attack of the groups, thereby hardening gelatin. In preparing a gelatin coating, an aqueous gelatin solution is usually used. In this case, however, it is inevitable that the coexisting water reacts with the hardening agent, thereby decomposing it and making it ineffective, because the water is nucleophilic to a certain extent. This tendency is particularly for hardening agents having a high gelatin-hardening speed. Accordingly, in order to harden gelatin efficiently and rapidly, it is necessary ta develop a hardening agent which reacts with graoups such as a carboxyl group and an amino group more quickly than with water; that is, a hardening agent having good selectivity in reaction with gelatin. Hardening agents such as N-carbamoylpyridinium salts and 2-sulfonyloxypyridinium salts have a disadvantage in that the efficiency of effectively using the hardening agent is poor, probably according to the fact that they are poor in the selectivity as defined above.
It has therefore been desired to develop a hardening agent which has a high efficiency, is high in the gelatin-hardening speed, and is highly soluble in water.